In recent years there has been a marked increase in the sophistication of downhole tools, and in particular, downhole tools deployed in the bottom hole assembly (BHA) of a drill string. A typical BHA commonly includes, for example, one or more logging while drilling (LWD) and/or measurement while drilling (MWD) tools. Such tools are well known to include various electronic sensors such as gamma ray sensors, neutron sensors, resistivity sensors, formation pressure and temperature sensors, ultrasonic sensors, audio-frequency acoustic sensors, magnetic sensors, acceleration sensors, and the like. LWD and MWD tools typically further include an electronic controller including at least one microprocessor and electronic memory. Moreover, a typical BHA further includes other tools, such as a telemetry tool, a formation sampling tool, and/or a rotary steerable tool, which include electronic controllers disposed to control, monitor, and record various tool functions during drilling.
It is also well known in the art that severe dynamic conditions are often encountered during drilling. Commonly encountered dynamic conditions include, for example, bit bounce, lateral shock and vibration, and stick/slip. Bit bounce includes axial vibration of the drill string, often resulting in temporary lift off of the drill bit from the formation (“bouncing” of the drill bit off the bottom of the borehole). Lateral shocks and vibrations are those which are transverse to the axis of the drill string and are often due to impact of the BHA with the borehole wall. Stick/slip refers to a torsional vibration induced by friction between drill string components and the borehole wall. Stick/slip causes rapid rotational acceleration and deceleration of the drill string and is known to produce instantaneous drill string rotation speeds many times that of the nominal rotation speed of the table. Bit bounce, lateral shock and vibration, and stick/slip are commonly recognized as leading causes of electronic failures in downhole tools. These electronic failures often result in costly trips (tripping the drill string in and out of the borehole) to repair or replace damaged tools and/or tool components.
Due in part to the above described dynamic conditions, the use of electronic sensors and controllers in downhole tools poses no small challenge. Moreover, it is commonly desirable to deploy MWD sensors (e.g., accelerometers, magnetometers, and gyroscopes) and certain LWD sensors (e.g., gamma ray sensors, neutron sensors, and density sensors) as close as possible to the longitudinal axis of the drill string. These sensors are typically deployed in a pressure housing that is centralized in the bore of a drill collar. In such configurations, it is typically desirable for the pressure housing to be both securely fixed to the drill collar and easily removable from the drill collar (e.g., for servicing the sensors between drilling operations). The centralizing mechanism should also be streamlined so as to enable drilling fluid to flow through the annulus between the inner surface of the drill collar and the outer surface of the pressure housing with minimal restriction.
Various centralizer configurations are known in the art. For example, one type of centralizer includes a pressure housing having metallic fins and/or rings sized and shaped to nearly contact the inner wall of the drill collar. This configuration may also employ o-rings for vibration dampening. While such an arrangement tends to adequately centralize the pressure housing(s), the necessary gap between the fins and drill collar tends to damage the inner surface of the collar under vibration and can actually amplify the shock and vibration seen by the electronics. Moreover, removal of the centralizer can be problematic due to mud packing between the ring and collar ID.
Another adaption of the metal finned or ring-type centralizer incorporates a wedge-locking device. These wedge-locking devices are typically energized using conventional screws. These devices often provide adequate locking and centralization of the pressure housing(s). However the screws have been known to loosen in service thereby unlocking the device and allowing motion of the device due to the downhole dynamic conditions. The screws have even been known to completely unscrew and fall down through the BHA to the bit where they can plug nozzles and causes drilling problems.
Another commercially available configuration utilizes molded rubber fins which are sized and shaped for a slight interference fit with the ID of the drill collar. While this arrangement tends to adequately dampen vibrations, installation and removal of the centralizer can be problematic due to the high coefficient of friction between rubber and steel. Moreover, the rubber fins are susceptible to tearing and chemical degradation which can lead to excessive movement of the pressure housings in service.
Pressure actuated, wedge locking centralizers are also known in the art. While such pressure actuated wedging mechanisms (provided by an inclined plane) increase the mechanical holding force of the stabilizer in the drill collar they tend to be bulky and therefore tend to significantly restrict the flow of drilling fluid through the drill collar. This restriction increases local fluid velocity and turbulence which in turn can lead to serious erosion and cavitation damage to the drill collar and pressure housing. Wedging mechanisms are further problematic in that a significant portion of the force exerted by the piston can be needed just to overcome the frictional forces between the wedge and the ramp and the movement needed between the wedge and the collar. The wedge style approach can also be problematic when trying to remove the system from the collar due to friction locking.
Therefore, there is a need in the art for an improved centralizer, for example, for centralizing and/or stabilizing pressure housings in a drill collar.